Yes, it does make sense. If many people don’t want to use their data minutes on voice calling, and they have a way to do voice calling anyway, then they’ll use the data minutes for areas they see the value - like surfing the web, doing social networking and watching YouTube. One other useful area would be video communications.
There clearly is a niche for video communications. Last year, Skype reported that half of their calls were video. It therefore makes sense, to me anyway, that mobile video communications would follow this same trend. First of all, if Skype users are used to it, they’d bring it to the mobile world. And if people are worried about their smartphone plans supporting video over the cellular network (or more accurately, how much this might cost them), and people want to do this, then they’d do it over Wi-Fi. Instead of bringing the baby to the camera that is mounted on the computer, bring the camera to the baby! So if you’re doing a basic voice call, do it on the mobile phone. But if you want to add video, do it over WiFi.